Some of the abbreviations used in this application are as follows:    CCCH Common Control Channel    DCCH Dedicated Control Channel    DRNC Drift Radio Network Controller    DTCH Dedicated Traffic Channel    FACH Forward Link Access Channel    IMSI International Mobile Subscriber Identity    PCCH Paging Control Channel    PCH Paging Channel.    PLMN Public Land Mobile Network    P-TMSI Packet Temporary Mobile Subscriber Identity    RACH Random Access Channel    RNC Radio Network Controller.    RNSAP Radio Network System Application Part    RNTI Radio Network Temporary Identity    RRC Radio Resource Control    TFCS Transport Format Combination Set    TFS Transport Format Set    TMSI Temporary Mobile Subscriber Identity    UE User Equipment    UMTS Universal Mobile Telecommunication System    UTRAN UMTS Terrestrial Radio Access Network
For clarification of common terms used in this document, an overview of certain cellular telecommunication system configurations is presented in the following.
Proposals for third-generation systems include UMTS (Universal Mobile Telecommunications System) and FPLMTS/IMT-2000 (Future Public Land Mobile Telecommunications System/International Mobile Telecommunications at 2000 MHz). In these plans cells are categorised according to their size and characteristics into pico-, nano-, micro- and macrocells, and an example of the service level is the bit rate. The bit rate is the highest in picocells and the lowest in macrocells. The cells may overlap partially or completely and there may be different terminals so that not all terminals necessarily are able to utilise all the service levels offered by the cells.
FIG. 1 shows an exemplary block diagram of a possible structure of a third generation cellular network. Such networks typically comprise a core network 50 connected to one or more radio access networks 40 (RAN). Such radio access networks are often referred to as UTRAN networks (UMTS Terrestrial Radio Access Network). The radio access networks typically comprise at least a plurality of base stations 20a,20b,20c (BS) for realizing the radio connections to mobile stations 10a,10b, and at least one radio network controller 30 (RNC) for controlling the base stations. The radio network controllers are connected to a mobile switching center (MSC) 60 in the core network.
More than one RNC may be involved with the connections of a single mobile station. Such a situation may result for example from handovers. For example, let us assume that mobile station 10a initiates connections while being in the cell of base station 20a, whereby the connections are initially in the control of RNC 30a. Later on the MS 10a moves to the cell of BS 20b, whereby the network performs a handover, in which the connections or at least some of them are transferred to BS 20b. In such a case, the connections now go from MSC 60 through RNC 30a to the RNC 30b, and finally to BS 20b. The two RNC's have slightly differing duties. The initial RNC is called the serving RNC (SRNC), and the second RNC is called the controlling RNC (CRNC). The second RNC is also often referred to as Drift RNC (DRNC). In the case of multi diversity connections, i.e. connections in which a single radio connection is effected with cooperation of multiple simultaneous connections via multiple base stations, there may be more than one controlling RNC's, each controlling one or more of the sub-connections of the multidiversity connection. The duties of a SRNC may be transferred to another RNC in order to optimize the connections within the cellular network. Such a process is called a serving RNC relocation.
Further, in the current specifications for third generation cellular systems, the interface between two RNC:s is called the Iur interface, and the interface between a MSC and a RNC is called the Iu interface. These interface names are used in this application.
Mobile stations, which in UMTS terminology are typically named as User Equipment (UE), need naturally be identified in some way within the UTRAN.
Temporary identifiers called Radio Network Temporary Identifiers (RNTI) are used as UE identifiers within an UTRAN and in signalling messages between the UE and the UTRAN. The RNTI identifiers are used and defined by the RNC's. Two types of RNTI are used in signalling messages between the UE and the UTRAN. One is used within and allocated by the SRNC and it is called the Serving RNC RNTI (s-RNTI). The other type is used within and allocated by a CRNC, when applicable, and it is called the Controlling RNC RNTI (c-RNTI). C-RNTI is often also called “Cell RNTI”.
A s-RNTI is allocated for all UEs having a RRC connection, it is allocated by the Serving RNC and it is unique within the Serving RNC. A s-RNTI is reallocated always when the Serving RNC for the RRC connection is changed. In addition, each RNC has an identifier, called the RNC identifier (RNC-ID). Together the RNC-ID and s-RNTI form a unique UE identifier within the UTRAN. For this unique UE identifier, the term UTRAN-RNTI (U-RNTI) may be used. A c-RNTI is allocated for an UE by each CRNC through which the UE is able to communicate on a DCCH channel. A c-RNTI is unique within the allocating CRNC. The signalling procedures in the 3GPP specifications allow C-RNTI to be unique also within one cell. A c-RNTI is always allocated when a new UE context is created in a CRNC.
Communication channels used for data transfer are grouped into two categories: common transport channels and dedicated transport channels.
Common transport channels where UE identification is performed by using the RNTIs comprise, according to current specifications, the following channels, among others:                Random Access Channel (RACH), which is used for transmission of relatively small amount of data, e.g. signalling for initial access or non-realtime dedicated control or traffic data,        Forward Access Channel (FACH), which is a downlink channel without closed-loop power control, and which is used for transmission of relatively small amounts of data, e.g. signalling (response) for initial access or non-realtime dedicated control or traffic data,        Paging Channel (PCH), which is a downlink channel used for broadcast of control information such as paging and notification information into an entire cell.        
According to current specifications the dedicated transport channel types comprise the following channel types, among others:                Dedicated Channel (DCH), which is a channel dedicated to one UE, and which can be used for uplink or downlink data transmission.        
Each transport channel has an associated Transport Format or an associated Transport Format Set. A Transport Format is a combination of various transmission parameters such as encodings, interleaving, bit rate and mapping onto physical channels. A Transport Format Set is a set of Transport Formats. For example, a variable rate DCH channel has a Transport Format Set i.e. one Transport Format for each available transmission rate, whereas a fixed rate DCH has a single Transport Format.
A third generation UE can be in many different states in relation to the network. If no connections are present, the UE is in the idle mode. When at least one signaling connection exists, the UE is in connected mode. The connected mode has two main states: an URA connected state and a cell connected state. The URA connected state may also be called URA_PCH state to reflect that UE is reachable only via paging channel (PCH). In the URA connected state, the position of the LIE is known on URA (UMTS Registration Area) level. An URA consists of a plurality of cells within a certain geographical area. In the cell connected state, the position of the UE is known in the cell level or in the active set level. All data transmission is effected in the cell connected state.
The cell connected state is further divided into a number of substates. Each state is associated with certain communication channels and other parameters. Therefore, the different states are typically denoted by the communication channels in use in the state. Further, the various communication channels have different properties. This collection of states and corresponding transport formats and channel types provide for different QoS levels, which can be provided for a UE.
According to the current specifications, the cell connected state has at least the following groups of substates:                In the DCH/DCH and DCH/DCH+DSCH substates, which may also be both called simply the CELL_DCH substate, a dedicated transport channel is allocated to the UE. In these states, the UE may transmit data up to the peak capacity that is currently granted to that UE.        In the RACH/FACH substate, which may also be called CELL_FACH substate, the UE monitors a FACH channel. It may transmit uplink control signals and may transmit small data packets on the RACH channel. Consequently, this state is used by UE's which do not need high amounts of transmission capacity.        In the PCH substate, which may also be called CELL_PCH substate, the UE listens to the PCH transport channel. The network needs to make a paging request on the PCCH logical channel in the known cell in order to initiate any downlink activity. For any uplink activity, the UE moves to the RACH/FACH substate.        
A UE may be transferred from DCH to RACH/FACH state for example as a result of the following RRC procedures:                Transport channel reconfiguration, in which a transport channel is changed from a dedicated to a common channel, for example for a NRT bearer.        Radio bearer (RB) release, in which at least one bearer is released, and the last remaining one is a non-real time (NRT) bearer which is currently not active or is which is configured to use common channels.        Physical channel reconfiguration, which procedure may assign, replace or release a set of physical channels used by an UE. A physical channel reconfiguration procedure may also change the used transport channel type and RRC state.        Radio bearer (RB) reconfiguration, in which parameters for a radio bearer or a signalling link (which may also be called Signalling Radio Bearer SRB) are reconfigured to reflect a change in required QoS level. A RB reconfiguration procedure may comprise for example changing of RLC parameters, changing of multiplexing priority for DTCH/DCCH or between DTCHs mapped to same DCH, changing of DCH scheduling priority, changing of TFS for DCH, change of TFCS, assigning or releasing of physical channel(s) and changing of used transport channel types.        
The signalling according to prior art in the case of the four previous procedures is similar: they are started by the serving RNC which sends a XXX message to the UE, which replies with a XXX Complete message, in which XXX refers to the particular procedure in question.
A UE is aware of its c-RNTI only when in RACH/FACH state, while c-RNTI is used as a UE identifier within UTRAN in all UE states. A further refinement of this mechanism allows the UTRAN to use a separate identifier for the UE within UTRAN, the drift RNTI (d-RNTI), instead of the c-RNTI. d-RNTI, as C-RNTI, is allocated by the controlling RNC in all the UE states, and it is used to identify the UE in the messages directed from SRNC to the CRNC, when needed.
S-RNTI together with the RNC-ID is used as a UE identifier in almost all CCCH messages and in UTRAN originated PCCH messsages on the air interface. The only exception is the initial RACH messages on CCCH where either a random number or some existing UE core network identifier such as IMSI, TMSI, or P-TMSI is used, because the s-RNTI is not allocated yet at that time. RNC-ID is used by a Controlling RNC to route the received uplink messages towards the Serving RNC.
C-RNTI is used as a UE identifier in DCCH/DTCH common channel messages on air interface. The main benefit of using c-RNTI instead of the combination of a s-RNTI and a RNC-ID in the air interface is to shorten the common channel messages and thus save capacity on common radio channels.
A problem with the current specifications for third generation cellular systems is signalling of the c-RNTI to the UE in some situations, such as:                when UE in RACH/PCH state is paged to move it to RACH/FACH state,        when UE is moved from DCH state to RACH/FACH state, and        when UE starts a CCCH procedure (e.g. Cell update) using a CRNC different from the SRNC.        
A known solution to these problems is to use the Cell Update procedure after packet paging or after a DCH-CCH transition for obtaining the new c-RNTI. However, this solution causes too much signalling on RACH/FACH channels. The capacity of the RACH/FACH channels is limited, and any signalling on these channels should be brought to a minimum. A better solution is therefore needed.